1. Field of Invention
The present invention relates to a chemical-mechanical polishing (CMP) station. More particularly, the present invention relates to a chemical-mechanical polishing station that has a belt-operated pad conditioner for improving the polishing action.
2. Description of Related Art
In the manufacturing of semiconductor devices, surface planarization is an important step in preparing a wafer for a high-resolution photolithographic processing operation. Only a smooth planar surface with little height variation can prevent diffraction of light from a light source when a pattern is transferred. In general, planarization techniques include spin-on-glass (SOG) method and chemical-mechanical polishing method. However, in the sub-half-micron device era, the spin-on-glass method of planarization is incapable of providing the degree of planarity required on a piece of wafer. Consequently, chemical-mechanical polishing has become the only method capable of providing global planarization up to the level of planarity required for fabricating devices in very-large scale integration (VLSI) or even ultra-large scale integration (ULSI) circuits.
FIGS. 1A and 1B are respective top and side views showing a conventional chemical-mechanical polishing station. As shown in FIGS. 1A and 1B, the station includes a polishing table 10, a wafer holder 11 for grasping a wafer 12, a polishing pad 13 over the polishing table 10, a tube 14 for carrying slurry 15 to the polishing pad 13, a liquid pump 16 for pumping slurry 15 into the tube 14, and a conditioner 17 for dressing the surface of the polishing pad 13. When the chemical-mechanical polishing station is carrying out a polishing action, the polishing table 10 and the wafer holder 11 independently spin in a pre-defined, opposite direction, for example, directions 18a and 18b respectively. The wafer holder 11, while gripping the backside 19 of the wafer 12, presses the front side 20 of the wafer 12 against the polishing pad 13. The liquid pump also works to continuously pump slurry 15 to the polishing pad 13 through the tube 14. The polishing action in a chemical-mechanical polishing operation relies on chemical reagents and abrasive particles suspended in the slurry. The reagents react chemically with molecules on the front surface 20 of the wafer 12 to form an easy-grind layer, while the abrasive particles of the slurry 15 help to remove pointed peaks within the easy-grind layer. By continuous chemical reaction and repeated mechanical abrasion, a highly polished and planar surface is ultimately formed on the wafer surface.
One major drawback of the aforementioned chemical-mechanical polishing station is that the conventional conditioner 17 is incapable of re-conditioning the surface of the polishing pad 13 to the original high degree of planarity and uniformity. FIG. 2A shows a top view and a side view of the first type of conventional chemical-mechanical mechanical polishing station. The particular station as shown in FIG. 2A has a model number IPEC-472. The station IPEC-472 has a polishing pad 30 located above the polishing table 32. Above the polishing pad 30, a wafer 34 and a conditioner 36 are placed.
When the wafer 34 is being polished, the conditioner 36 will move forward and backward following the directions as indicated by the arrow 38 so that the polishing pad 30 can be re-conditioned back into a planar surface. FIG. 2B shows a top view and a side view of the second type of conventional chemical-mechanical polishing station. The particular station as shown in FIG. 2B has a model number AMAT-Mirra. The station AMAT-Mirra has a polishing pad 40 located above the polishing table 42. A wafer 44 and a conditioner 46 are placed above the polishing pad 40.
When the wafer 44 is being polished, the conditioner 46 swings to the left and right according to the directions indicated by the arrow 48 so that the polishing pad 40 can be re-conditioned into a planar surface. FIG. 2C is a diagram showing a portion of the tracks left by the respective conditioners when the polishing pads of the polishing stations as shown in FIGS. 2A and 2B are re-conditioned. As seen in FIG. 2C, the tracks produced by the conditioner are not uniform. For example, some places are rarely touched by the conditioner, thereby leading to under-conditioning of the polishing pad as indicated by the relatively blank region in area 54. On the other hand, some areas have been repeatedly scoured causing over-conditioning of the polishing pad. An example is the area 56 near the crossing point between two trajectories. Consequently, only a few places such as track area 52 are normally conditioned.
FIG. 3A shown a top view and a side view of the third type of conventional chemical-mechanical polishing station. The particular station as shown in FIG. 3A has a model number SpeedFam Auriga. The station SpeedFam Auriga has a polishing pad 60 located above a polishing table 62. Above the polishing pad 60, a wafer 64 and a conditioner 66 are placed. The conditioner 66 has a diamond ring structure, for example.
When the wafer 64 is being polished, the conditioner 66 sweeps over the peripheral regions of the polishing table 62 to recondition the polishing pad 60 into a flatter surface. FIG. 3B is a cross-section showing the resulting profile of the polishing pad after the polishing pad is conditioned by the conditioner as shown in FIG. 3A. In FIG. 3B, units of the horizontal axis are marked in centimeters (cm). It is obvious from FIG. 3B that after the conditioner has been used for a while because the pad profile of the polishing pad 60 is highly irregular and non-uniform. In fact, the polishing pad 60 has a central bulge region and a sagging edge region. Consequently, the polishing surface of the polishing pad for polishing the silicon wafer 64 becomes highly irregular. Hence, wafer 64 near the central region is polished more while the peripheral region is polished less.
FIG. 4A shows a top view and a side view of the fourth type of conventional chemical-mechanical polishing station. The particular station as shown in FIG. 4A has a model number Cybeq-IP8000. The station Cybeq-IP8000 has a polishing pad 70 located above a polishing table 72. A wafer 74 and a conditioner 76 are placed above the polishing pad 70. When the wafer 74 is being polished, the condiitoner 76 will sweep over the peripheral regions of the polishing table 72 to recondition the polishing pad 70 into a flatter surface.
FIG. 4B is a cross-section showing the resulting profile of the polishing pad after the polishing pad is conditioned by the conditioner as shown in FIG. 4A. It is obvious from FIG. 4B that after the conditioner has been used for a while pad profile 78 of the polishing pad 70 becomes highly irregular and non-uniform. Consequently, the polishing surface for polishing the silicon wafer 74 becomes uneven. Hence, wafer 64 near the central region is polished less while the peripheral region is polished more (just opposite to the situation in FIG. 3B).
In summary, all four conditioners 36, 46, 66 and 76 employed by various models of polishing stations cause non-uniformity of the polishing pad due to uneven distribution of scouring tracks (as indicated by FIG. 2C). Therefore, after the polishing pad has been reconditioned by one of the conditioners for awhile, problematic height difference can be found all across the pad surface (as shown in FIGS. 3B and 4B). Hence, ultimate wafer profile produced by the polishing station can be highly irregular, thus severely compromising the quality of wafer finish.
In light of the foregoing, there is a need to provide an improved conditioner for conditioning the polishing pad in a chemical-mechanical polishing station.